An amorphous alloy thin strip is produced by rapidly cooling an alloy in a molten state. Processes such as the centrifugal rapid cooling process, the single-roll process, the twin-roll process and the like are known as the methods for producing thin strips. In such a process, a thin strip or a thin wire is produced by ejecting molten metal through an orifice or the like onto the inner or outer surface of a rapidly rotating metal drum and thus rapidly solidifying the molten metal. An amorphous alloy excellent in magnetic, mechanical and/or corrosion properties is obtained by suitably selecting the alloy composition thereof.
Such an amorphous alloy thin strip is viewed as a promising industrial material for various applications due to the excellent properties thereof. As a material for the iron core of a power transformer, a high frequency transformer or the like, in particular, an iron-base amorphous alloy thin strip, for example that of an Fe—Si—B system, is used for the reason that it has a low core loss, a high saturation magnetic flux density, a high magnetic permeability, etc.
An iron-base amorphous alloy thin strip that has electrically insulating films of oxide or the like formed on the surfaces, for the purpose of improving the magnetic properties when it is used as a material for an iron core, is known. In an iron core for a transformer formed by winding a thin strip or laminating thin strip sheets, the insulating coating films have the effects of improving electrical insulation between the layers of the iron core and reducing the eddy current loss caused by crossover magnetic flux.
The present inventors have disclosed in Japanese Unexamined Patent Publication No. H11-300450 an iron-base amorphous alloy thin strip produced by rapid cooling and solidification and having an ultra-thin oxide layer of an adequate thickness at least on one of the surfaces, and another thin strip having a segregation layer containing P and/or S in the lower portion of an oxide layer similar to the above.
The present inventors have also disclosed in Japanese Unexamined Patent Publication No. 2000-309860 an iron-base amorphous alloy thin strip having a segregation layer containing one or more of As, Sb, Bi, Se and Te in the vicinity of the interface between an ultra-thin oxide layer and the amorphous mother phase. In addition, they have disclosed in Japanese Unexamined Patent Publication No. 2000-313946 an iron-base amorphous alloy thin strip having an ultra-thin oxide layer of a bilaminar structure, and another similar thin strip having one or more of P, As, Sb, Bi, S, Se and Te segregating in the second lamina of the oxide layer on the side of the mother phase.
When a wound iron core transformer or a laminated iron core transformer is fabricated with such an amorphous alloy thin strip as mentioned above, usually, the thin strip is wound toroidally to form a wound iron core or many sheets of the thin strip are piled to form a laminated iron core, and thereafter the iron core undergoes annealing while a direct current magnetic field is imposed in the direction of a magnetic circuit. The purpose of annealing is to improve a magnetic flux density by creating magnetic anisotropy in the direction of the imposed magnetic field and to lower a core loss by reducing strain existing in the thin strip.
When an annealing temperature is low in the above treatment, magnetic anisotropy is hardly created and therefore a magnetic flux density does not improve; what is worse, strain is not removed and therefore a core loss is not lowered either. However, when an annealing temperature is low, the embrittlement of a thin strip resulting from annealing is mitigated.
On the other hand, when an annealing temperature is high, a magnetic flux density is improved and, at the same time, strain is removed sufficiently and therefore a core loss is reduced, but the embrittlement of a thin strip becomes significant. The cause of the embrittlement of a thin strip resulting from annealing has not been clarified yet, but it is estimated that the embrittlement is caused by the fact that atoms, which have been arranged comparatively randomly, are locally rearranged into orderly structures during rapid cooling and solidification. When an annealing temperature is still higher, a thin strip crystallizes and the excellent soft magnetic properties peculiar to an amorphous material are not retained any longer.
Therefore, there is a certain optimum temperature in the annealing of an iron core. In such an annealing treatment, however, as the weight and the volume of an iron core increase, temperature unevenness is more likely to occur at different portions of the iron core during heating after it is charged into a heat treatment furnace. The temperature unevenness can be reduced by taking sufficient time during heating and cooling, but this lowers productivity.
As measures for improving such an annealing process, various methods have been proposed so far: for example, Japanese Unexamined Patent Publication No. S63-45318 discloses a method wherein a heat insulating material is attached around the inner and outer circumferences of an iron core and thus temperature differences in the iron core during cooling are minimized. Ideally, it is desirable to improve a thin strip itself so that temperature unevenness may not cause an adverse effect even when it occurs. However, there has not so far been any iron-base amorphous alloy thin strip that can reduce the performance deterioration caused by temperature unevenness at different portions of an iron core in an annealing process.
In view of the above situation, the present inventors have invented an iron-base amorphous alloy thin strip capable of securing excellent soft magnetic properties and suppressing the embrittlement of the thin strip even when temperature unevenness occurs at different portions of the iron core during annealing or a lower annealing temperature is applied, by adding P, to an amount in a specified range, to an alloy having a composition in the range wherein the amounts of Fe, Si, B and C are regulated, and have applied the invention as Japanese Patent Application No. 2001-123359 (hereinafter referred to as “the prior invention”).
Each of the iron-base amorphous alloy thin strips disclosed in the aforementioned patent publications contains the following elements as a part of each desirable chemical composition: P and/or S in the range from 0.0003 to 0.1 mass % in the case of Japanese Unexamined Patent Publication No. H11-300450; one or more of As, Sb, Bi, Se and Te in the range from 0.0003 to 0.15 mass % in the case of Japanese Unexamined Patent Publication No. 2000-309860; and one or more of P, As, Sb, Bi, S, Se and Te in the range from 0.0003 to 0.15 mass % in the case of Japanese Unexamined Patent Publication No. 2000-313946.
As stated in the description of the aforementioned prior invention, iron-base amorphous alloy thin strips containing P have been disclosed in Japanese Unexamined Patent Publication Nos. S57-185957, H8-193252, H9-202946, H9-202951, H9-268354 and H11-293427. However, each of the patent publications is different from the prior invention in chemical composition, and does not reduce the performance deterioration caused by temperature unevenness.
Meanwhile, when such an iron-base amorphous alloy thin strip is cast, high purity iron such as electrolytic iron has been used as iron source for the reason that a low core loss is not secured if impurity elements are contained therein, and other reasons. In relation to this, the present inventors have disclosed in Japanese Unexamined Patent Publication No. H9-202946 an Fe—Si—B—C system amorphous alloy thin strip having a specific chemical composition and containing, in mass, 0.008%≦P≦0.1%, 0.15%≦Mn≦0.5%, and 0.004%≦S≦0.05% as impurity elements. In such a thin strip, not only a core loss is improved but also the permissible amounts of Mn and S as impurity elements are increased by containing a small amount of P as stated above (0.1 mass % P corresponds to 0.16 atomic % P, approximately), and, as a result, an inexpensive steel produced through ordinary steelmaking processes can be used as an iron source.
A steel produced through ordinary steelmaking processes contains, as impurity elements, besides Mn and S mentioned above, various elements originating from deoxidizing agents, refractory materials, different grades of steel sticking to steelmaking vessels, and so on. Among those elements, the elements easily combining with O, N or C and forming precipitates, such as Al, Ti and Zr, accelerate the crystallization of an amorphous alloy thin strip during casting, and, for this reason, a steel containing the possible least amounts of these elements has so far been used.
With regard to Al and Ti, it is described that a very small addition amount of either Al or Ti causes the crystallization in the surface layers of a thin strip and the deterioration of a core loss in the Proceedings of the 4th International Conference on Rapidly Quenched Metals, 957 (1981) regarding Al and in the Journal of the Japan Institute of Metals, Vol. 52, No. 7, 733 (1988) regarding Ti.
Further, Japanese Unexamined Patent Publication No. H4-329846 discloses that the deterioration of product properties can be inhibited by adding 0.1 to 1.0 mass % Sn and/or 0.01 to 0.05 mass % S in the event of using a low purity raw material containing one or more of Al, Ti and Zr by 0.01 mass % or more. However, the patent publication also discloses that the addition of Sn and/or S causes the deterioration of embrittlement. Further, as seen in Example of the patent publication, even with the addition of Sn, a core loss is still at a poor level of 0.15 W/kg or more in W13/50.